(i) Field of the Invention
The present invention relates to a resin for urethane lenses having good optical properties and heat resistance, to lenses comprising the resin, and to processes for the preparation of the resin and the lenses.
(ii) Description of the Related Art
Plastic lenses are lighter and less breakable as compared with inorganic lenses, and are also dyeable. Therefore, in recent years, the application of plastic lenses to optical elements such as spectacles and cameras has increased rapidly.
A resin which presently has been widely used for this purpose can be obtained by the radical polymerization of diethylene glycol bis(allylcarbonate) (hereinafter referred to as "D A C"). This kind of resin is characterized by having excellent impact resistance, being lightweight, and having excellent dyeing properties, good workability such as cutting and polishing, and the like.
With regard to this resin, however, its refractive index is lower (n.sub.D =1.50) than that of the inorganic lenses (n.sub.D =1.52). Accordingly, in order to obtain the optical characteristics equal to those of glass lenses, it is necessary to increase the thickness of the center and periphery of the plastic lenses as well as their curvature, so that the thickness of the plastic lenses inevitably increases on the whole. For this reason, the resin for lenses having a higher refractive index is desired.
Furthermore, polyurethane resins are known as resins for lenses having a high refractive index which can be obtained by the reaction of an isocyanate compound and a hydroxy compound such as diethylene glycol (Japanese Patent Laid-open No. 57-136601), the reaction of the isocyanate compound and a hydroxy compound having a halogen atom such as tetrabromobisphenol A (Japanese Patent Laid-open No. 58-164615), and the reaction of the isocyanate compound and a hydroxy compound having a diphenyl sulfide skeleton (Japanese Patent Laid-open No. 60-194401).
Moreover, the same assignee as in this application has already offered, as resins for lenses having a high refractive index, polyurethane resins and lenses comprising the resins obtained by the reaction of an isocyanate compound and a hydroxy compound having a sulfur atom (Japanese Patent Laid-open No. 60-217229) and the reaction of the isocyanate compound and a polythiol compound (Japanese Patent Laid-open Nos. 60-199016, 62-267314 and 63-46213).
The lenses comprising these known resins have a higher refractive index than the lenses using D.A.C, but the degree of its improvement is not sufficient. Additionally, in these resins, compounds having many halogen atoms or an aromatic ring in each molecule thereof are used so as to improve the refractive index, and therefore they have some drawbacks, for example, poor weathering resistance and increased specific gravity.
Moreover, in the plastic lenses offered by the present inventors, the refractive index is still poor; dispersion is more perceptible considering the value of the refractive index as compared with glass lenses; and heat resistance is insufficient in a subsequent step of dyeing, coating and the like. For these reasons, further improvement is demanded.